This invention relates in general to the field of amplifiers and in particular to microwave power amplifiers.
Microwave power amplifiers of increasing efficiency are important in many applications, including those related to radios. For example, in the field of mobile radio equipment, microwave power amplifiers with a high efficiency make it possible to reduce the size and power dissipation of radio transmitters.
By using class F amplification, an efficiency of about 70% has been achieved. The class F operation is performed under a special terminating condition in which the amplifier output terminal is short circuited at even harmonics and open-circuited at odd harmonics. Ideal switching is possible with such a configuration, resulting in 100% drain efficiency. Such high efficiency performance is possible only when using second harmonics in the amplifier.
For higher microwave frequencies, however, it is increasingly difficult to obtain high efficiencies, even with a class F amplifier. Construction to close tolerances and adjustment at the output termination are required to accurately establish the short circuit condition. In addition, class F amplifiers need to be biased to class AB, and therefore are not applicable to high power amplifier applications.
The harmonic reaction amplifier (HRA) is capable of realizing the terminating condition necessary for achieving ideal switching operation and can provide high efficiency at higher frequencies. The HRA achieves ideal class B operation with zero quiescent current without incurring efficiency losses. Output losses are kept to a minimum in a high efficiency and high power amplifier having over approximately 10 watts (W) of output power.
The HRA is described in Nishiki, S. and Nojima, T., "Harmonic Reaction Amplifier-a Novel High-Efficiency and High-Power Microwave Amplifier," IEEE MTT-S Digest, DD-5, pp. 963-966, 1987 and in Nishiki, S. and Nojima, T., "High Efficiency Microwave Harmonic Reaction Amplifier," IEEE MTT-S Digest, pp. 1007-1010, 1988. The HRA described by Nishiki and Nojima uses two signal reject filters and two second harmonic reject filters coupled with a variable second harmonic path. It would be desirable to simplify the Nishiki and Nojima HRA configuration, simplifying analysis and implementation. It would also be desirable to provide an improved method and apparatus for harmonic reaction amplification with lower insertion loss, resulting in a high efficiency of operation and a small size HRA.